Recommendation engines aren’t the biggest issue. People will figure out how to fins what they want, and be generally happy with that, if looking is easy enough.
The big issue is that “join the fediverse” is a really, really shitty and incomplete recommendation. It’s like “join the blogosphere!”
And “join Mastodon” or “join Lemmy” is bad, too. It’s like asking them to “join Joomla”.
You need to point people to the specific website they should join, and that website has to already have what they’re looking for. People aren’t interested in building something.
They just want to consume.
What we really need is for people to put up topic focused sites and promote them as their own thing, not jusy “lemmy”. So many specific interests still have very active forums dedicated to them, populated by the kind of people who want to ask queations aboht and discuss the things they have interest or expertise in, but who aren’t into things like Reddit.
The fediverse is perfect for places like that. Places where you can focus on your primary interest, but also look over the fence. But all anyone wants to do is put up general interest sites and whine about there being more than one “gaming” forum.